This application is based on Japanese Patent Application No. 9-66793 filed on Mar. 19, 1997, the entire contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to a semiconductor device and a method of using the device, and more particularly to a semiconductor device having a quantum well structure and a method of using such a device.
b) Description of the Related Art
A quantum well structure is formed by inserting a semiconductor thin film having a narrow bandgap between two semiconductor layers having a wide bandgap. The semiconductor layer having narrow bandgap is called a quantum well layer, and the semiconductor layer having a wide bandgap is called a barrier layer. A multiple quantum well structure is formed by alternately stacking a semiconductor layer having a narrow band gap and a semiconductor layer having a wide band gap.
The operation principle of a light modulator using a multiple quantum well structure will be described hereinunder.
FIG. 11A shows a band energy distribution in a quantum well as a function of position in the thickness direction. A quantum well layer L2 is sandwiched between barrier layers L1 and L3. In FIG. 11A, a kinked line Ec is a conduction band edge, a kinked line Ev is a valence band edge, a broken line el0 shows a ground level of electron, and a broken line hh0 shows a ground level of heavy hole.
In the state without applying a bias electric field to such a quantum well structure, a wave function efo of electron at the ground level elo of the conduction band and a wave function hf0 of heavy hole at the ground level hh0 of the valence band are both symmetrical with the center of the well layer L2. The peak position of the electron wave function ef0 is equal to that of the heavy hole wave function hf0. If light having an energy corresponding to a difference Eg or higher between the electron ground level el0 and the heavy hole ground level hh0 is incident upon the quantum well structure, electron-hole pairs are generated and light is absorbed.
In FIG. 12A, a curve a0 shows a wavelength dependency of an absorption coefficient in the state shown in FIG. 11A.
FIG. 11B shows the band structure when the quantum well structure shown in FIG. 11A is applied with a bias electric field along the right direction as viewed in FIG. 11A. As the electric field is applied, the band edges are slanted. The band edges of the quantum well layer L2 are slanted upper right so that electrons distribute to the left and holes distribute to the right. As the band edges are slanted, the difference Eg between the electron ground level el0 and the heavy hole ground level hh0 becomes small so that the absorption wavelength moves to the longer wavelength side (red shifts) corresponding to a lower energy side.
The wave functions of electron and hole shown in FIG. 11A have the same peak position if an electric field is not applied. As an electric field is applied, the wave functions shown in FIG. 11B shift in opposite directions, and as the intensity of the electric field increases, the overlap portion of the wave functions reduces. Reduction in the overlap portion of the wave functions means a lowered absorption coefficient xcex1. Therefore, as the electric field is applied to the quantum well structure, the curve a0 shown in FIG. 12A moves to the longer wavelength side, and the height of the curve a0 lowers.
A curve a1, shown in FIG. 12A shows the wavelength dependency of the absorption coefficient xcex1 in the state shown in FIG. 11B. As shown, the absorption coefficient xcex1 rises on the longer wavelength side than a wavelength xcex0, and light having the wavelength xcex0 is absorbed. The intensity of light having the wavelength xcex0 can therefore be modulated by controlling an electric field applied to the quantum well structure.
A charp parameter is defined by xcex94n/xcex94k, where xcex94n is a change in the real part of a complex refractive index relative to a change in the electric field applied to a light modulator, and xcex94k is a change in the imaginary part thereof. The change xcex94k in the imaginary part of a complex refractive index is related to a change rate xcex94xcex1 of an absorption coefficient relative to a light intensity, by a relationship of xcex94k=xcexxcex94xcex1/4xcfx80. A wavelength change xcex94xcex of a light pulse generated by modulating an applied electric field is given by xcex94xcex/xcex2=xe2x88x92(xcex94n/xcex94kxc3x97dS/dt)/(4xcfx800S), where S is a light intensity which changes with time, c0 is a light velocity in vacuum. If the charp parameter is not zero, the wavelength changes with modulation of the light intensity.
A conventional electric fieldxe2x80x94absorption type light modulator has a large positive value of the charp parameter in a transparent state with a weak applied electric field and a small absorption coefficient, and takes a negative value in a non-transparent state with a strong applied electric field and a large absorption coefficient. With a conventional light modulator, the charp parameter is positive in most of an applied electric field which provides a light intensity higher than a certain level. In this case, as a light pulse rises increasing its light intensity, the wavelength of modulated light once shifts to the shorter wavelength side and then recovers its initial wavelength, whereas as a light pulse falls decreasing its light intensity, the wavelength of modulated light once shifts to the longer wavelength side and then recovers its initial wavelength. Namely, in the region having a high intensity of the light pulse, the wavelength of modulated light moves from the shorter wavelength side to the longer wavelength side during the period between the pulse leading and trailing edges.
A quartz single mode optical fiber prevailing in the field of optical fiber communications has so-called dispersion characteristics that a propagation velocity (group velocity) of a light pulse changes with the wavelength of propagating light. Although the wavelength of about 1.55 xcexcm minimizes a propagation loss, this wavelength band has so-called abnormal dispersion characteristics that the longer the wavelength becomes, the lower the group velocity becomes. In using this optical fiber in the 1.55 xcexcm wavelength band, since a conventional light modulator has a wavelength change, the propagation velocity at the pulse trailing edge moving to the longer wavelength side becomes lower than that at the pulse leading edge moving to the shorter wavelength side, and the pulse width is broadened during optical fiber transmission. Therefore, the higher the modulation speed becomes and the narrower the pulse width becomes, discrimination between two adjacent light pulses becomes more difficult and transmission errors are likely to occur.
If an optical fiber having such characteristics is used near at the wavelength of 1.55 xcexcm, a charp parameter is generally desired to be as small as possible. If a negative charp parameter is realized, the light pulse width can be narrowed during optical fiber transmission as opposed to the conventional example. Therefore, transmission errors can be suppressed even if a faster signal is transmitted over a long distance.
It is an object of the present invention to provide a semiconductor device capable of generating a light pulse suitable for long distance transmission.
According to one aspect of the present invention, there is provided a semiconductor device comprising: a quantum well lamination structure having at least one quantum well layer and at least two barrier layers alternately laminated, the quantum well layer forming a quantum well for electron and hole and the barrier layer forming a potential barrier relative to electron and hole, wherein a thickness of the quantum well layer and a height of the potential barrier of a valence band at the interface between the quantum well layer and the barrier layer are set so that the number of quantum level relative to hole on the valence band side of the quantum well layer is two or three in the state that the intensity of an electric field generated in the quantum well layer is zero; and means for applying an electric field in the quantum well lamination structure in a thickness direction.
As a bias voltage is applied to the quantum well layer, the band edges are slanted so that the energy gap lowers. Therefore, the light absorption edge wavelength red-shifts. This red shift can be used for a light modulator. As a bias voltage is applied, the wave functions of electron and hole confined in the quantum well shift in opposite directions. There is some range in which a value of an overlap integral of the wave function of electron at the ground level of the conduction band and the wave function of hole at the higher level of the valence band reduces as the bias voltage is increased. By using this range, the charp parameter of the quantum well layer can be made negative.
According to another aspect of the present invention, there is provided a semiconductor device comprising: a quantum well lamination structure having at least one quantum well layer and at least two barrier layers alternately laminated, the quantum well layer forming a quantum well for electron and hole and the barrier layer forming a potential barrier relative to electron and hole, the quantum well layer having a ground level and a first higher order level for hole on the valence band side thereof; an n-type semiconductor layer in contact with one surface of the quantum well lamination structure; and a p-type semiconductor layer in contact with the other surface of the quantum well lamination structure, wherein a difference between a transition wavelength between a ground level for electron on a conduction band side of the quantum well layer and the ground level for hole on the valence band side and a transition wavelength between the ground level for electron on the conduction band side of the quantum well layer and a first higher order level for hole on the valence band side is 40 nm or smaller over the whole range from a flat band state of the quantum well lamination structure with a normal bias voltage being applied between the n-type semiconductor layer and the p-type semiconductor layer to a state with a certain reverse bias voltage being applied.
By switching between the light transparent state and the light non-transparent state in the range where the transition wavelength difference is 40 nm or shorter, the charp parameter can be made negative in the dynamic operation range.
According to another aspect of the present invention, there is provided a semiconductor device comprising: a quantum well lamination structure having at least one quantum well layer and at least two barrier layers alternately laminated, the quantum well layer forming a quantum well for electron and hole and the barrier layer forming a potential barrier relative to electron and hole, the quantum well layer having a ground level and a first higher order level for hole on the valence band side thereof; an n-type semiconductor layer in contact with one surface of the quantum well lamination structure; and a p-type semiconductor layer in contact with the other surface of the quantum well lamination structure, wherein a value of an overlap integral of a wave function of electron at the ground level on the conduction band side of the quantum well layer and a wave function of hole at the first higher order level on the valence band side reduces as the reverse bias voltage is increased, over the whole range from a state that the bias voltage is not applied between the n-type semiconductor layer and the p-type semiconductor layer to a state with a certain reverse bias voltage being applied.
The charp parameter can be made negative in the dynamic operation range by switching between the light transparent state and the light non-transparent state in the range where a value of an overlap integral of a wave function of electron at the ground level on the conduction band side of the quantum well layer and a wave function of hole at the first higher order level on the valence band side reduces as the reverse bias voltage is increased.
According to another aspect of the present invention, there is provided a semiconductor device comprising: a quantum well lamination structure having at least one quantum well layer and at least two barrier layers alternately laminated, the quantum well layer forming a quantum well for electron and hole and the barrier layer forming a potential barrier relative to electron and hole, the quantum well lamination structure having a ground level and a first higher order level for hole on the valence band side of the quantum well layer; an n-type semiconductor layer in contact with one surface of the quantum well lamination structure; a p-type semiconductor layer in contact with the other surface of the quantum well lamination structure; and voltage applying means for selectively applying a first bias voltage or a second bias voltage across the n-type semiconductor layer and the p-type semiconductor layer, wherein a difference between a transition wavelength between a ground level relative to electron on a conduction band side of the quantum well layer and the ground level for hole on the valence band side and a transition wavelength between the ground level for electron on the conduction band side of the quantum well layer and a first higher order level for hole on the valence band side is 40 nm or smaller over the whole range from a state applied with the first bias voltage to a state applied with the second bias voltage.
The charp parameter can be made negative in the dynamic operation range by setting the first bias voltage applied state to the transparent state and the second bias voltage applied state to the non-transparent state.
According to another aspect of the present invention, there is provided a semiconductor device comprising: a quantum well lamination structure having at least one quantum well layer and at least two barrier layers alternately laminated, the quantum well layer forming a quantum well for electron and hole and the barrier layer forming a potential barrier relative to electron and hole, the quantum well lamination structure having a ground level and a first higher order level for hole on the valence band side of the quantum well layer; an n-type semiconductor layer in contact with one surface of the quantum well lamination structure; a p-type semiconductor layer in contact with the other surface of the quantum well lamination structure; and voltage applying means for selectively applying a first bias voltage or a second bias voltage across the n-type semiconductor layer and the p-type semiconductor layer, wherein a value of an overlap integral of a wave function of electron at the ground level on the conduction band side of the quantum well layer and a wave function of hole at the first higher order level on the valence band side reduces as the reverse bias voltage is increased, over the whole range from a state applied with the first bias voltage to a state applied with the second bias voltage.
The charp parameter can be made negative in the dynamic operation range by setting the first bias voltage applied state to the transparent state and the second bias voltage applied state to the non-transparent state.
According to another aspect of the present invention, there is provided a method of using a semiconductor device, wherein the semiconductor device comprising: a quantum well lamination structure having at least one quantum well layer-and at least two barrier layers alternately laminated, the quantum well layer forming a quantum well for electron and hole and the barrier layer forming a potential barrier relative to electron and hole, the quantum well lamination structure having a ground level and a first higher order level for hole on the valence band side of the quantum well layer; an n-type semiconductor layer in contact with one surface of the quantum well lamination structure; and a p-type semiconductor layer in contact with the other surface of the quantum well lamination structure, wherein a difference between a transition wavelength between a ground level for electron on a conduction band side of the quantum well layer and the ground level for hole on the valence band side and a transition wavelength between the ground level for electron on the conduction band side of the quantum well layer and a first higher order level for hole on the valence band side is 40 nm or smaller over the whole range from a state applied with a first bias voltage across the n-type semiconductor layer and the p-type semiconductor layer to a state with a second bias voltage larger than the first bias voltage, the second bias voltage being a reverse voltage, and the method comprising the step of applying a light flux to the quantum well structure and switching between a state that the light flux is transmitted by applying the first bias voltage and a state that the light flux is not transmitted by applying the second bias voltage.
The charp parameter can be made negative in the dynamic operation range.
According to another aspect of the present invention, there is provided a method of using a semiconductor device, wherein the semiconductor device comprising: a quantum well lamination structure having at least one quantum well layer and at least two barrier layers alternately laminated, the quantum well layer forming a quantum well for electron and hole and the barrier layer forming a potential barrier relative to electron and hole, the quantum well lamination structure having a ground level and a first higher order level for hole on the valence band side of the quantum well layer; an n-type semiconductor layer in contact with one surface of the quantum well lamination structure; and a p-type semiconductor layer in contact with another surface of the quantum well lamination structure, wherein a value of an overlap integral of a wave function of electron at the ground level on the conduction band side of the quantum well layer and a wave function of hole at the first higher order level on the valence band side reduces as the reverse bias voltage is increased, over the whole range from a state applied with a first bias voltage to a state applied with a second bias voltage larger than the first bias voltage, the second bias voltage being a reverse voltage, and the method comprising the step of applying a light flux to the quantum well structure and switching between a state that the light flux is transmitted by applying the first bias voltage and a state that the light flux is not transmitted by applying the second bias voltage.
The charp parameter can be made negative in the dynamic operation range.
As described above, a semiconductor device used as a light modulator is provided which has a negative charp parameter xcex94n/xcex94k in the range from the transparent state to the non-transparent state. Accordingly, even if an optical fiber having wavelength dispersion can be used for long distance transmission.